1. Field of the Invention
This invention relates to a sheet conveying apparatus having a plurality of electrodes disposed to attract and convey a sheet by an electrostatic force, and an image recording apparatus provided with the same.
2. Related Background Art
An ink jet recording apparatus having a sheet conveying apparatus of a conventional electrostatic attraction type will hereinafter be described with reference to the accompanying drawings. FIG. 7 is a schematic cross-sectional view of an image recording apparatus according to the conventional art, and FIGS. 8A and 8B are a plan view and an enlarged cross-sectional view, respectively, of a sheet conveying apparatus. The image recording apparatus 50 shown in FIG. 7 has a recording head 51 for effecting image recording, and a conveying belt 52 which is a sheet conveying apparatus of an electrostatic attraction type. The recording head 51 is of a line type in which discharge openings are arranged over a width substantially equal to the width of a sheet conveyed, and is capable of effecting high-speed image formation as compared with a serial type in which a recording head is carried on a carriage and is caused to scan.
However, because of the high speed, discharged ink is unfixed immediately after recording, and there is the undesirable possibility of an image being disturbed if a recording surface is contacted with a sheet conveying apparatus for holding the sheet. So, in such an apparatus, design is made such that the back side of the recording surface of the sheet is attracted to the conveying belt 52 provided with electrostatic attracting means and the sheet is conveyed without the conveying belt contacting with the recording surface.
The conveying belt 52 is a circular ring-shaped belt member passed over a driving roller 58 and a plurality of supporting rollers 59, 60 and 61, and is rotatively driven to thereby convey the sheet which is a recording medium such as paper or a plastic sheet. As the electrostatic attracting means in this conveying belt 52, there is, for example, one as shown in FIG. 8A wherein a group of electrodes (hereinafter referred to as the comb-teeth electrodes 53 and 54) comprising strip-like electrodes of different polarities alternately disposed are formed and as shown in FIG. 8B, electrically conductive brushes 55 as electricity supplying means are provided on the opposite ends of the conveying belt 52. In such a construction, when one of the comb-teeth electrodes 53 and 54 is connected to earth potential and a predetermined voltage is applied to the other to thereby produce a potential difference, an attracting force can be provided by an electrostatic force.
However, the comb-teeth electrodes 53 and 54 are embedded in the conveying belt 52 and therefore, the thickness of the conveying belt 52 and the hardness of the material thereof vary in the electrode portions and these factors appear as the irregularity of speed in the driving roller 58 portion or the like, and the accuracy of sheet conveyance is reduced. Also, generally the electrode portion is worked by printing or the welding of the electrode materials, but it is difficult to effect such working on an endless belt and therefore, the conveying belt is formed by working it into a band-like belt, and thereafter joining the ends of the belt into an endless shape.
Therefore, the conveying belt 52 does not become a seamless belt of good mechanical accuracy and particularly, the joining portion adversely affects an image in the application thereof to a recording apparatus for the reason that the degree of smoothness thereof is lowered, or causes an inconvenience such as jam or rubbing by the floating of a sheet. Therefore, usually it is necessary to detect the joining portion of the conveying belt, and control the belt so that no sheet may be placed on the joining portion, and a marker 56 has been printed on the conveying belt 52 or a reflecting member has been attached to the conveying belt 52 to thereby effect detection by the use of an optical sensor 57. However, it has required much labor and cost in the manufacture to print the marker 56 on a surface discrete from the electrode pattern or stick it in the form of a seal. Also, the position at which the marker is stuck on the conveying belt has been limited.